The fair place to meet is Las Vegas, NV — the city closest to the midpoint of Los Angeles and San Diego. From the farther side that’s about 5 hr 33 min of driving.
Recommended midpoint
Las Vegas, NV
From Los Angeles
4 hr 47 min
228 mi to Las Vegas
From San Diego
5 hr 33 min
265 mi to Las Vegas
Los Angeles has the shorter trip; the split is off by about 46 min. The alternatives below can even it out. Los Angeles and San Diego are about 111 miles apart.
Los Angeles and San Diego are about 111 miles apart by road. Split the difference and you arrive near Las Vegas, the city closest to the halfway point between them. That puts roughly 4 hr 47 min of driving on the Los Angeles side and 5 hr 33 min on the San Diego side — the fairest single meeting point among the cities near the middle.
At this range, Las Vegas works for a same-day meetup: close enough from both Los Angeles and San Diego to meet for lunch or an afternoon and still be home by evening.
If Las Vegas doesn't have what you're after, Phoenix and San Jose are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Los Angeles and San Diego.
Las Vegas, colloquially shortened to Vegas, is the most populous city in the U.S. state of Nevada and the county seat of Clark County. It is the 24th-most populous city in the United States, with 641,903 residents at the 2020 census. Read more →
Los Angeles (LA) is the most populous city in the U.S. state of California, and the commercial, financial, and cultural center of Southern California. Read more →
San Diego is a city on the Pacific coast of Southern California, adjacent to the Mexico–United States border. It is the eighth-most populous city in the U.S. Read more →
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.
Estimates use straight-line distance and typical road speeds; real drive times vary with route and traffic.